The paper industry has been practicing waste paper recycling to regenerate usable cellulosic fiber for paper making for many decades. In these processes, ink removal from the waste paper pulp is improved using a suitable deinking composition. By controlling the deinking process, a recycling mill can affect the properties of the pulp such as the brightness and can improve the usability of the cellulosic fiber for paper manufacturing.
Deinking consists of a series of complex chemical and physical processes. These events include but are not limited to ink detachment, ink dispersion, ink collection, ink transport, and removal of inks from the waste paper pulp slurry. Each of these microprocesses have different surface and interfacial demands within the recycling operation in order to efficiently and effectively deink waste paper and produce quality paper.
Conventionally, two general approaches have been employed to remove ink in order to produce the deinked fiber after repulping. These two approaches are flotation and wash deinking. Often, processes contain both flotation and wash deinking and can be referred to as combination deinking processes. The underlying chemical and physical requirements to successfully deink are different for wash, flotation, and combination deinking processes.
More specifically, flotation/washing combination deinking refers to a deinking process wherein the ink released from the fiber is separated from the cellulosic fibers primarily through the flotation devices or flotation cells of the recycling process prior to passing the pulp through washing stages. Alternatively, washing/flotation combination deinking refers to a deinking process wherein the ink released from the fiber is separated from the cellulosic fibers primarily through the washing devices of the recycling process prior to passing the pulp through flotation stages. Wash deinking, on the other hand, refers to a deinking process wherein the ink released from the fiber is separated from the cellulosic fibers in the washing stages.
Flotation processes are fundamentally different than washing processes. This difference is partly because the ink size and hydrophobicity are important for favorable separation. Flotation methods of ink removal generally involve passing air bubbles through an aqueous system containing dispersed cellulose fiber that is most often produced via a repulping process. The resulting repulped cellulose fiber slurry having therein additives added either before, during, or after repulping. As the air bubbles rise within the fiber slurry and carry the ink particles with them, they generate foam which is enriched in ink that is subsequently removed from the flotation cell. The amount of foam that is typically favored in flotation deinking systems is an amount which can be collected by skimming, decanting, or other means, and which transports with it an enriched concentration of ink while minimizing the amount of other solids such as fibers that are rejected.
Flotation deinking generally utilizes different surfactants than washing because the resulting surface properties and size of the ink particles that is beneficial for flotation deinking is different than is desirable for wash deinking. Examples of traditional nonionic surfactants that may be used in the flotation deinking process include alkylene oxide adducts of fatty alcohols, alkylphenols, fatty acids, and alkanolamides. Such nonionic surfactant deinking agents may be used either by themselves or in combination with one another and can also be blended with nonalkoxylated fatty acids and fatty alcohols. Flotation deinking has historically relied on a high pH pulping liquor to remove ink from the fiber and facilitate efficient usage of pulper bleach additives.
Wash deinking typically requires fine dispersion of ink. The ink and fiber are uniformly distributed throughout the slurry, and foaming or bubble formation, though present, is not particularly desired. The objective in the washing process is to release the ink from the fiber into an aqueous medium and then separate the fiber from the aqueous medium. Thus, the washing method comprises repulping, preferably under low-foaming conditions, of secondary fiber in an ink-removing aqueous medium whereby the ink (and other non-cellulosic contaminants, if present) is mechanically and/or chemically removed as desired from the fiber. The repulping step is typically followed by dilution and/or screening. Certain surfactants are known to those skilled in the art to modify interfacial properties successfully for wash deinking. These surfactants, however, also modify the ink into a hydrophilic dispersed state that favors the washing method.
Washing and flotation processes both depend on the proper use of surfactant. Depending on the relative contributions and characteristics of the hydrophilic and hydrophobic portions of the surfactant molecule, the surfactant's interaction with the ink will vary as ink particles will be rendered either more hydrophilic for washing purposes or more hydrophobic for flotation. The different natures of wash surfactant and flotation surfactant can lead to inefficiency in combination deinking systems. The deinking mechanism for washing is quite different from that for flotation and, therefore, they often require the use of deinking compositions having different properties.
Typically, either a flotation-derived or washing-derived deinking chemistry is utilized in a given deinking system. At some point in either process, the deinked, repulped waste paper is often passed through a series of fine cleaners and/or screens where the small particulate contaminants (e.g., sand and grit) are removed. Additional processing stages may be required such as, for example, dispersion, to reduce the particle size of any contaminants, or a special cleaning stage with special cleaners designed to remove specific contaminants.
The chemistry involved in traditional deinking very often involves addition of caustic soda in the repulper to increase the pH, often greater than 9 and sometimes greater than 10. Increasing the pH, however, can cause yellowing and darkening of the waste paper stock, especially when the waste paper contains groundwood or mechanical pulps. To counteract this undesirable darkening effect, a bleaching additive can be added to increase the whiteness and brightness of the pulp. The deinked waste paper is then held in storage until it is eventually fed to a papermaking machine.
Surfactant-based deinking aids, especially nonionic surfactants, can be good ink detachment agents. Nonionic surfactants can also assist in ink collection. If not properly chosen, however, these additives can also hinder ink collection in flotation containing processes. It is well known that traditional fatty acid soaps are efficient ink collectors in alkaline systems (pH greater than 9), although these soaps can lead to deposit concerns later in the process.
Traditional nonionic surfactants used in deinking are typically formed through alkoxylation, which includes, but is not limited to, ethylene oxide and proplylene oxide. A base hydrophobe, often an alcohol or fatty acid, is reacted with ethylene oxide or propylene oxide or both. The order of addition can affect the final structure and properties. Alternately, propylene and ethylene oxide are reacted together to form block or random copolymers. These reactions can require specialized equipment.
U.S. Pat. No. 5,643,409 discloses a deinking agent obtained by the addition of an alkylene oxide to a polyfunctional alcohol having 1 to 10 carbon atoms and/or a polyfunctional fatty acid having 1 to 10 carbon atoms. U.S. Pat. No. 5,801,135 discloses nonionic surfactants obtained by the random-addition and/or block-addition reaction of an alkylene oxide having 2 to 4 carbon atoms to alcohols, amines or fatty acids each having hydrogens activated for additions, or fatty esters thereof. EP 0488306 discloses deinking agents made by esterifying dicarboxylic acids such as an alkylsuccinic acid or an alkenylsuccinic acid with a polyoxyalkylene alkyl ether or a polyoxyalkylene alkenyl ether. U.S. Pat. No. 5,736,622 describes a deinking collector which includes a polyester obtained by reaction between a polyalkylene glycol, di and/or tricarboxylic acid and/or anhydrides thereof as well as a saturated fatty acid with 12-18 carbons. U.S. Pat. No. 5,120,397 describes a deinking composition which is the reaction product obtained by reacting a natural oil or fat, or a reaction product of a natural oil or fat with glycerin, with a hexahydric alcohol to obtain an ester mixture and subsequently reacting the ester mixture with ethylene oxide and propylene oxide. U.S. Pat. No. 5,228,953 describes an additive comprising a polyglycol and a complex mixture of esters formed from C1-C20 alcohols esterified with phosphoric acid for flotation deinking of waste paper. U.S. Pat. No. 5,304,316 describes a deinking agent obtained by reacting an ethoxylated and propoxylated fatty acid or an ethoxylated and propoxylated incomplete ester of a polyhydric alcohol with a dicarboxylic acid or an anhydride thereof.
Traditional surfactant choices in deinking applications are often the result of blending multiple surfactants. A limited number of hydrophobes are available which are reacted with various amounts of alkoxylating agents. As such, there is a limitation to the properties than can be obtained by blending a fixed number of surfactants. In addition, manufacturing restrictions and/or economics can prohibit customization for individual applications.
Therefore, there is a need to develop and prepare functionally optimized deinking structures without the use of the standard alkoxylation protocol which can be produced economically and wherein changes can be easily accomplished to maximize the performance within a given, often single, molecular structure.